Standards ETSI ETS 300 132-2 and ANSI T1.315 specify an allowable amount of audio band emissions (up to 20 kHz) that may be conducted from a telecom shelf onto DC power feeders supplying the shelf. Typically, the main source of these audio band conducted emissions is the forced air cooling system, which comprises fans or blowers drawing a current pulsed at less than 1 kHz. The invention suppresses the audio band conducted emissions resulting from the pulsed current sufficiently to comply with the stated standards.
FIGS. 1 and 2 show two prior art approaches of limiting audio band emissions conducted onto DC power feeders supplying a telecom shelf. In the first approach, shown in FIG. 1, a large passive L-C filter 10 is placed between the power feeder connection point on the shelf and a high bandwidth DC-DC converter 11 that supplies the fans or blowers 12 of the shelf""s cooling system. The passive filter limits the amount of audio band emissions that are conducted from the DC-DC converter 11 back onto the DC power feeders. In the second approach, shown in FIG. 2, a low bandwidth active filter 14 is placed between the high bandwidth DC-DC converter 13 and the fans or blowers 15. In this case the low bandwidth active filter limits the audio band emissions conducted through the DC-DC 13 converter onto the DC power feeders. In both approaches, the audio band emissions caused by the pulsed current used to drive the fans or blowers are sufficiently reduced before they are conducted back onto the DC power feeders, thereby enabling the telecom shelf to meet the aforementioned standards.
Regarding the first prior art approach, shown in FIG. 1, the main problem with this approach is that the physical size of the components required for the passive filter 10 is large, and therefore they occupy more than a desirable amount of shelf space, which is usually at a premium.
Regarding the second prior art approach, shown in FIG. 2, although the active filter 14 occupies less space than the passive filter 10 of FIG. 1, the approach still requires an amount of shelf space that could be further reduced. Additionally, a more complex thermal management solution is typically required due to resultant heating of a series pass element in the active filter.
Both approaches share a problem that is inherent to the high bandwidth DC-DC converters commonly used to regulate the DC voltage applied to the fan or blowers.
Typically, these converters are switch-mode power converters, which have an analog frequency control loop.
The frequency response of this control loop is set by the inherent characteristics of the power converter output filter and switch modulator, as well as the compensation network applied to the feedback error amplifier. In a power converter, it is very common to set the control loop bandwidth to a point well above the pulsed current frequency drawn by the fan or blower. In so doing, the power converter will react to any in band AC load applied to its output, thereby reflecting the AC current signature to its input and onto the power feeders for the shelf. Consequently, additional filtering is required to suppress the audio band noise reflected onto the power feeders.
The present invention solves the above discussed problem by providing a switch-mode direct current-to-direct current power converter and method for supplying power from a direct current power feed to a forced air cooling system of a communication system, the forced air cooling system unit having an air forcing unit. The fans/blowers units require a DC power source only. Internal to the fan/blower unit is a motor drive circuit that derives from the DC source an AC voltage necessary to drive the fan/blower. The resulting load current signature from the fan/blower drawn from the DC source is an AC waveform with a DC offset. The power converter includes an input adapted to couple to the direct current power feed and an output adapted to electrically drive the air forcing element. In a preferred embodiment, an analog frequency control loop controls the amount of the direct current provided to the air forcing element, the control loop having a response bandwidth which is lower in frequency than that of the alternating current drawn from the output by the air forcing element. Preferably, the analog frequency control loop comprises a compensation network for providing the response bandwidth. Furthermore, the frequency of the alternating current is preferably below 1 kHz and a capacitor is coupled to the output.